Bandwidth tailoring of SAW ladder filters via IDT configuration for 5G multi-band monolithic integration

This paper presents a bandwidth-tailoring surface acoustic wave (SAW) ladder filter for monolithic multiband integration in 5G systems. The proposed approach suppresses excessive bandwidth by controlling the effective electromechanical coupling coefficient (k2eff) of the resonators through modification of the interdigital transducer (IDT) electrode configuration. Specifically, changing the IDT electrode polarity sequence modifies the electric field distribution and thereby enables controllable tuning of k2eff. Finite-element method simulations show that changing the IDT polarity sequence from 1:1 to 1:3 reduces the k2eff of the series resonator from approximately 21% to 10.5%, with only a negligible shift in the series resonance frequency. Based on this approach, ladder filters targeting the n77, n78, and n79 bands were fabricated on an X-cut LiNbO3/SiO2/SiC platform. The fabricated filters exhibit minimum insertion losses of 0.62, 1.04, and 2.03 dB, corresponding to 3 dB bandwidths of 600, 280, and 370 MHz, respectively. The Bode-Qmax of the modified series resonators changes from 470 to 381 for n78 and from 252 to 218 for n79. These results demonstrate that the IDT configuration method provides a practical approach for tailoring filter bandwidth in monolithically integrated tunable SAW filters.